Computer input and other pointing devices, such as electronic mice, convert physical movement into movement of a cursor or other image across a computer screen. Previously, many such devices utilized mechanically driven encoder wheels and other moving components to detect direction and magnitude of motion, and to then convert that information into data for communication to a computer or other device. Optical surface tracking offers an improved method of motion detection. Instead of encoder wheels rotated by a ball rolling across a surface, an array of photo-sensitive elements generates an image of a desktop (or other supporting surface) portion when light from an associated illumination source (such as a light emitting diode) reflects from the desktop or other surface. Subsequent images are compared, and based on the correlation between images, the magnitude and direction of mouse motion may be determined. Exemplary optical tracking systems, and associated signal processing techniques, include those disclosed in commonly owned U.S. Pat. Nos. 6,172,354, 6,303,924 and 6,373,047.
FIG. 1A schematically shows various components of an existing optical tracking system in a computer mouse 1a. Mouse 1a (shown in phantom lines) moves across a surface 2a such as a desk top or a table. A region 3a of the bottom surface of mouse 1a is either transparent or open so that light may reach a portion of the surface 2a (the “target area” T) and be reflected back to an image sensor 7a. A light source 4a inside of mouse 1a, which is typically a LED, is selectively turned on and off so as to controllably illuminate the target area T. Light from LED 4a reflects from the target area and is collected and focused by a lens 5a through an aperture 6a. Light passing through aperture 6a strikes a photo-sensing surface of an image sensor 7a. Image sensor 7a then forms (sometimes in connection with other components) an image of the target area T (or a portion thereof). Typically, image sensor 7a is attached to a Printed Circuit Board (PCB) 8a, only a portion of which is represented in FIG. 1A.
Although an improvement over mechanically-tracking types of motion sensing systems, optically-tracking systems present a new set of challenges. The light source, lens, image sensor and other components must be properly positioned with respect to one another. Permissible tolerances for this positioning are generally closer than tolerances associated with assembly of mechanical tracking components. The illumination pattern is also important. It is preferable for the tracking surface to be diffusely and evenly illuminated, with “bright” and “dead” spots minimized. It is also important for the illumination pattern to be as uniform as possible from device to device.
Existing optically-tracking devices direct light to a target area in various manners. In some cases, the LED is positioned between the image sensor and the target surface. Although simple in some respects, this configuration can add additional steps to an assembly process, and can also be a potential source of errors because of improper alignment. Moreover, many commercially available LEDs are not optimized for direct illumination of a near-field object (such as a target area of a tracking surface), resulting in less-than-desirable illumination patterns. Other configurations use a light guide to direct light from a LED to the target area. One such configuration is shown in FIG. 1B, in which components 1b–8b are generally similar to components 1a–8a of FIG. 1A. In the configuration of FIG. 1B, however, light from LED 4b is transmitted to the target area T via light guide 9b. Typically, light guide 9b is formed from light-transmissive material such as glass or plastic. The light from LED 4b enters light guide 9b and reflects from the internal surfaces of the material, and then exits from an exit face e to illuminate the target area. Although addressing some problems of direct illumination, the configuration of FIG. 1B still fails to illuminate the target area as desired, causing dead spots and/or bright spots. The presence of dead and bright spots leads to Fixed Pattern Noise (FPN) and degrades imaging accuracy. Accordingly, there remains a need for improved methods and systems of illuminating target areas in optically tracking devices.